1. Field of the Invention
The present invention relates to print-head drive control techniques for inkjet printers, and more specifically to a technique for reducing variation in discharge position between blocks caused in time-division driving of a print head and increasing the accuracy of registration adjustment.
2. Description of the Related Art
Recently, technologies of recording apparatuses have been considerably advanced in accordance with the development of personal computers. The recording apparatuses are structured to record an image on a sheet of paper on the basis of image information. In such a recording apparatus, a recording method that has recently been attracting attention most is an inkjet recording method in which recording is performed by discharging ink toward a sheet of paper from a print head. The inkjet recording method is advantageous in that high definition images are recorded at a high speed, and is superior to other recording methods in various points such as running cost and quietness.
On the other hand, registration adjustment is necessary for high-quality printing. With regard to methods for correcting displacements between landing points of ink drops discharged from nozzles of different colors or displacements between landing points of ink drops discharged from nozzles of the same color while scanning in opposite directions in the case of bidirectional printing, various techniques are known and are incorporated in many products.
In a typical print head that has recently been used, nozzles are arranged in a staggered pattern, as shown in FIGS. 2A and 2B. The figures show examples in which 320 black (BK) nozzles and 64 COLOR nozzles (for each color) are provided. In FIG. 2A, the BK nozzles are arranged in two lines: an EVEN nozzle line including 160 nozzles with even numbers on the left and an ODD nozzle line including 160 nozzles with odd numbers on the right. The nozzles of the same color are arranged in two lines with a predetermined pitch py in the y direction, and the two lines are separated from each other by a distance px corresponding to a predetermined number of pixels. The nozzles in the two lines are shifted from each other by a distance corresponding to (py/2) in the y direction. According to this arrangement, the print resolution of a single line can be doubled by adjusting the discharge timing of the two nozzle lines.
In such a case, it is necessary to correct differences in landing points of ink drops of the same color between rasters and differences in discharge direction between the EVEN and ODD nozzle lines. These differences are not only caused due to individual differences in the manufacturing process but are also influenced by the ink composition, the history such as discharge frequency, the environment in which recording is performed, etc. Accordingly, even when discharge timing of a certain head is set such that no difference in the landing points occurs under a predetermined condition, it does not mean that this setting can be applied to every case. More specifically, variations caused in the manufacturing process of the head are, of course, adjusted before shipping, and further adjustments must be performed in accordance with the usage history, etc., as necessary. A method of registration adjustment for correcting the above-described differences is suggested in Japanese Patent Laid-Open No. 2001-129985.
A typical print head is driven by a driving method in which nozzles arranged in a single line along a column direction (y direction) are divided into a plurality of nozzle groups and are driven at different timings in units of the nozzle groups. This method is described in detail in Japanese Patent Laid-Open No. 2000-071433. According to this method, i.e., time-division driving of the nozzles, the speed and stability in the ink-supplying process are increased and power consumption required for the discharge is reduced.
FIGS. 3A and 3B are tables showing the examples in which a BK nozzle line and a COLOR nozzle line, respectively, are each divided into 16 blocks. As shown in the tables, each block includes nozzles disposed at an interval of 32 nozzles. When only the EVEN nozzle line is considered, each block includes nozzles disposed at an interval of 16 nozzles. Since the nozzles separated from each other at certain intervals are classified into the same block, each nozzle is not easily influenced by the operations of the adjacent nozzles.
With regard to methods of registration adjustment, a method of shifting print data for each column by a plurality of pixels or half a pixel in units of print resolution, a method of shifting the print timing from a reference timing by a predetermined time, etc., are known. The method of shifting the print data for each column by a plurality of pixels is performed for correcting the displacements between the landing points of ink drops discharged from nozzles of different colors or roughly adjusting the displacements between landing points of ink drops discharged from nozzles of the same color while scanning in opposite directions in the case of bidirectional printing. As shown in FIG. 15, when the print resolution is 600 dpi in the scanning direction of a print head, the amount of shift can be set in units of 600 dpi. In the case of half-pixel shift, the amount of shift can be set in units of 0.5 pixels corresponding to the print resolution, that is, in units of 1200 dpi in the above-described case. In the method of shifting the print timing from the reference timing by a predetermined time, the print timing is shifted within the time period for a single column in units corresponding to a basic clock used for operating a printer system. This is performed for correcting small displacements due to the individual differences caused in the manufacturing process or the environment in which recording is performed.
However, although print start positions can be shifted by the above-described methods, the time for discharging from the blocks included in a single column in the time-division driving is constant. For example, when the print speed (carriage speed) is 40 inches/sec and the resolution is 600 dpi, the time for discharging from all of the nozzle lines in a single column is calculated as follows:Tcolumn=( 1/40(inch/sec))/600(dpi)=41 μsecThe discharge time for each column is typically set by reading a scaler provided along the moving direction of the carriage with an encoder on the carriage. Accordingly, the discharge time for each column is constant in a print area where the carriage speed (scan speed of the recording head) is constant. The driving time for each block in a single column is obtained by dividing the discharge time for each column by the number of blocks. For example, in the above-described case in which each column is divided into 16 blocks, the driving time for each block is calculated as follows:Tblock=Tcolumn/16 (blocks)=2.60 μsec.
As described above, since the time for each column and that for each block are evenly set on the basis of the reference timing, when the registration adjustment is performed, the discharge start time is shifted while the discharge time for each column is maintained constant. In the known structure, the discharge time for each column in the raster direction is determined by the print speed of the carriage and the print resolution. However, in practice, the print speed of the carriage is limited to several modes in view of the optimum values of the discharge frequency of the print head, and therefore the discharge time for each column is determined depending on the print resolution.
Recently, volumes of ink drops have been reduced to achieve high-quality printing like film photographs, and it has become possible to discharge ink drops of an extremely small volume such as 1 pl to 2 pl (picoliters). Such an extremely small ink drop forms a very small dot on a sheet of paper. Accordingly, in the time-division driving in which the discharge time differs between the blocks, there may be a case where the discharge positions of the nozzles are not aligned along a line. In conventional print heads, the volumes of ink drops are relatively large, such as 20 pl to 50 pl, and the dots formed by the ink drops overlap each other on a sheet of paper, so that the displacements between the dots in each column caused in the time-division driving are not noticeable.
In the above-described case, the time difference between the first block and the 15th block is calculated as about 39 μsec by multiplying the time for each block by a factor of 15. Although displacements between ink drops of 2 pl cannot be noticed by human eyes, they appear as a discernible striped pattern when the displacements occur over the entire area of the image on the sheet.